A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. Conventionally each trace records one seismic source activation, so there is one trace for each live source-receiver combination. In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2-D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3-D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
An ideal marine seismic source would cover the entire frequency band of interest, and only the frequency band of interest for seismic surveying, i.e., about 1-100 Hz or even higher (e.g., up to 300 Hz) depending on the survey objectives. Swept-frequency sources are of increasing interest as an alternative to conventional sources due to their ability to control the bandwidth of their signal sweep. However, in practice it is very difficult to build a single swept-frequency source that covers this entire range.
Conventional marine seismic sources are located in water and thus only radiate P waves. For some applications other wave modes such as surface waves can also provide valuable information. Surface waves travel horizontally through the shallow sedimentary section, and thus allow imaging of shallow features that may be difficult to image using more vertically traveling P waves. Although P waves generated by conventional marine seismic sources do convert into other wave modes when they pass from water into sediment, they do so only weakly. Ocean-bottom sources have been developed and deployed in an effort to better generate such waves. However, the use of ocean-bottom sources has been very limited because of their high cost of operation.
Low-frequency surface waves have recently shown promise for imaging marine near-surface velocity anomalies, which may represent geohazards such as shallow gas. Low-frequency surface waves are particularly difficult to generate using active marine seismic sources, but are an abundant component of the seismic noise background in shallow-water marine settings.
Thus, interest has turned in recent years to the use of passive seismic surveys which do not utilize a conventional/active seismic source. In a passive survey, the seismic receivers continuously record the ambient seismic signal/noise for a relatively long period of time (e.g., multiple hours, days, etc.). Then, using autocorrelation, cross correlation, or other techniques for performing seismic virtual-source interferometry well known to those of ordinary skill in the art, the data records that contain the generally unintelligible raw data signal can be processed to provide images of the subsurface.
The advantages of such an approach are clear. First, the environmental and logistical impact of such a survey is much less than one that utilizes an active seismic source, which is typically an air gun (in a marine environment) or dynamite or vibrators (in a land survey).
Second, the seismic noise background is often rich in the low frequencies that are difficult to generate using active sources. In a shallow-water marine environment, the low-frequency seismic noise background is particularly rich in surface waves. Virtual-source interferometry of low-frequency surface-wave passive data has recently shown promise for detecting shallow velocity anomalies and/or changes in shear-wave splitting magnitudes and polarizations such as those that might be associated with movements of shallow gas or fluids, or changes in properties of the subsurface such as porosity or its state of stress. Generating such low-frequency surface waves using an active source would conventionally require a large ocean-bottom source, which would typically be infeasible both due to its cost and because of the operational risk of damage to facilities and infrastructure.
Third, the expense of sources, particularly ocean-bottom sources, may also be avoided. Ocean-bottom receivers, which are generally much less expensive to deploy, become virtual ocean-bottom sources.
Finally, with passive data no effort needs to be made to create artificial sources. In principle, data can be recorded continuously for arbitrarily long periods. This is obviously of great benefit if the goal is real-time or near-real-time surveillance.
To create good images from ambient noise alone requires that very large volumes of data be collected. For example, 2000 four-component receivers recording at a sample rate of 2 milliseconds generate 54 gigabytes of data per hour, or one and a quarter Terabytes every day. Experience has shown that a few hours of recording is typically not enough to produce a good image. Ideally, several days' worth of data would be used. Such a volume of data is difficult to store, transport, or transmit, especially on an ongoing basis, which severely limits the usefulness of the method for real-time surveillance. As a result, the oil industry has acquired only a few large passive datasets. Most of the existing datasets only span a few hours, barely long enough to produce a useful result. For continuous surveillance, the datasets are much larger than the examples given above. Continuing the previous example, an array of 2000 four-component receivers recording at a sample rate of 2 milliseconds would generate 459 Terabytes of raw seismic recordings per year.
Another problem is that conventional interferometry requires the noise to be “uniformly distributed”. The noise on any given hour or day may or may not be suitably distributed, depending on field operations, ocean conditions, the weather, etc. One solution is to use a dense receiver array that enables any non-uniform distribution to be corrected in processing (e.g., see Stork, Christof, US 2010/0054083, Measuring and modifying directionality of seismic Interferometry data). However, dense receiver arrays are more expensive than sparse ones which tends to limit the usefulness of this technique. A simpler solution is to simply record for a longer period of time since a recording that is acquired over a longer period of time is more likely to have noise that is uniformly distributed.
Heretofore, as is well known in the seismic acquisition and processing arts, there has been a need for a system and method that provides a more efficient method of acquiring and processing passive seismic data that does not suffer from the disadvantages of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.